Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are becoming more important as power sources to be mounted in vehicles that utilize electricity as a motor driving power supply, as power sources to be used in personal computers, portable terminals, and other electronic products, and so on. In particular, lithium ion secondary batteries that are lightweight and provide a high energy density are preferred as high output power sources to be mounted in vehicles.
In nonaqueous secondary batteries such as lithium ion secondary batteries, however, a part of the nonaqueous electrolyte can decompose during charging, and a solid electrolyte interphase (SEI) film, which is a coating formed from the decomposition products thereof, can be formed on the surface of the negative electrode active material (for example, natural graphite particles). The SEI coating plays a role in protecting the negative electrode active material, but because it is formed by expending the charge carrier (for example, lithium ions) in the nonaqueous electrolyte, (in other words, through being immobilized in the SEI layer the charge carrier can no longer contribute to capacity), if the amount of the coating is too great, it becomes a factor that contributes to a drop in capacity retention rate (decrease in cycling characteristics).
To counteract this problem, an additive that decomposes at or above a predetermined charging voltage and forms a coating on the surface of the negative electrode active material can be included in the nonaqueous electrolyte so that it will form a stable coating beforehand on the surface of the negative electrode active material in place of the SEI coating.
The use of such an additive is noted in Patent Literature 1 as prior art. Patent Literature 1 describes a technique wherein an attempt is made to form a coating on the surface of the negative electrode active material by injecting a nonaqueous electrolyte containing a specified additive, performing an initial charging, and thereafter further re-injecting the above nonaqueous electrolyte containing the specified additive, and recharging the battery.